The coiled tubing is also referred to as coiled tubing or flexible oil tubing, which is a tubing made of low carbon alloy steel with excellent flexibility and can be winded round a roller. A roll of coiled tubing is several thousand meters long and can perform many operations instead of the conventional oil tubing. The coiled tubing operation equipment has characteristics of pressurized operation and continuous trip, the equipment volume is small, the operation cycle is short, and the cost is low.
As compared with the prior art, operating with the coiled tubing has many advantages. In particular, during a sidetracking operation, the coiled tubing could achieve a continuous circulation mud without making a connection, thereby decreasing the trip time and the operation cycle, avoiding any possible blowout or drill-jamming accident caused by making a connection, and improving the operation efficiency and security.
Although the coiled tubing drilling technique has many advantages, the aspects such as the adaptability of the coiled tubing technique to the drilling, the coiled tubing drilling process, and related tools still need to be further studied. Thus, the coiled tubing drilling technique is not practically applied in the drilling industry until 1990s. Along with the pressure attenuation of some old oilfields and the discovery of special oil-gas reservoirs, the sidetracking operation of the coiled tubing is increasingly advantageous. As to an old well, a sidetracking through the coiled tubing is an effective method to exploit the remaining reserve and reduce the cost, while the casing windowing is an indispensable key link of the sidetracking. Meanwhile, due to its distinction from the conventional drilling, the coiled tubing cannot accomplish the location and orientation of the down-hole tool at the wellhead by rotating the coiled tubing through a rotary table or a top drive device, while the window sidetracking operation is a complex down-hole operation. Therefore, how to safely and reliably complete the whole window sidetracking operation becomes the focus of the current industry.